The present invention relates to methods for tuning gas turbine fuel nozzle assemblies and particularly relates to methods for resizing premix fuel inlet holes for supplying gaseous fuel for premixing with air within the nozzle assemblies.
In land based gas turbines, a fuel nozzle typically comprises a subassembly of generally concentric tubes defining a central passage for supplying diffusion fuel gas and a pair of concentric passages for supplying premix fuel gas. Spaced from and surrounding the subassembly is an inlet flow conditioner for directing and confining a flow of inlet air past a plurality of circumferentially spaced vanes carried by the subassembly. The vanes are in communication with the concentric fuel gas supply passages. Particularly, the vanes include outer premix holes and inner premix holes for supplying gas from the respective passages for mixing with the inlet air. The gas fuel mixture is swirled by the vanes downstream of the fuel inlet holes for subsequent combustion.
The gas fuel composition and Wobbe Index at site locations determine the fuel gas nozzle exit velocity requirement which in turn is dependent upon the fuel gas supply hole size. Where the supply holes are too large, for a given gas composition and Wobbe Index, nozzle dynamics become a concern. For example, if the gas composition changes, these concerns become real and the nozzle assembly must be retuned to preclude those dynamic concerns.
In accordance with an example of the present invention and in a fuel nozzle assembly for a gas turbine having a plurality of circumferentially spaced vanes with holes for flowing fuel for premixing with air within the nozzle assembly, there is provided a method of tuning the fuel nozzle assembly by changing the diameter of the premix fuel holes in the vanes. To accomplish this, the existing holes are reformed to a predetermined diameter. Plugs are inserted into the reformed holes and secured to the vanes. Holes are formed through at least three of the plugs to diameters less than the diameter of the existing holes. Thus, the original holes are resized to provide smaller holes with consequent desired tuning effects.